A conventional inkjet printing system includes a printhead, an ink supply that supplies liquid ink to the printhead, and an electronic controller that controls the printhead. The printhead ejects ink drops through a plurality of orifices or nozzles toward a print medium, such as a sheet of paper, so as to print onto the print medium. Typically, the orifices are arranged in one or more arrays such that properly sequenced ejection of ink from the orifices causes characters or other images to be printed upon the print medium as the printhead and the print medium are moved relative to each other.
Typically, the printhead ejects the ink drops through the nozzles by rapidly heating a small volume of ink located in vaporization or firing chambers with small electric heaters, such as thin film resisters. Heating the ink causes the ink to vaporize and be ejected from the nozzles. Typically, for one dot of ink, a remote printhead controller typically located as part of the processing electronics of a printer, controls activation of an electrical current from a power supply external to the printhead. The electrical current is passed through a selected thin film resister to heat the ink in a corresponding selected vaporization chamber.
Inkjet technology is based on injecting ink through a nozzle by heating it to the boiling point. A bubble of air is formed that pushes some ink out of the nozzle of the printhead. As the ink is expelled from a nozzle, it leaves a small void of mass in the vaporization chamber from which it left. This creates a vacuum that pulls fresh ink into the vaporization chamber. With fresh ink in the vaporization chamber, the nozzle is ready to fire another ink drop. A subsystem known as the ink delivery system (IDS) is responsible for supplying the vaporization chamber with a fresh supply of ink. An IDS pump is used to provide pressure to supply ink to the vaporization chamber.
In ink demanding applications, if the ink pressure is too low, the vaporization chambers will not be refilled fast enough causing printhead starvation. One consequence of printhead starvation is that print quality degrades dramatically as some of the nozzles stop ejecting ink and white lines show up in the printed image. A second consequence of printhead starvation is that nozzles heat up very fast, which heats the printhead. Eventually, the printhead can experience a thermal shutdown resulting in the print job being stopped.
Typical solutions to these problems involve setting and maintaining a constant ink pressure that will allow the maximum flow rate of ink through the printhead. The maximun flow rate of ink through the printhead is determined by firing all nozzles at the maximum frequency. Most of the time, however, printheads do not fire all the nozzles at once. A more typical scenario is that only 5% to 20% of the nozzles fire most of the time and very rarely do 100% of the nozzles fire at once. Therefore, the IDS pump is producing an IDS pressure that is greater than required most of the time.
In order to maintain a higher pressure, the IDS pump runs more often and under greater load conditions than is really required most of the time. This in turn shortens the life of the IDS pump and decreases the overall reliability of the printing system. The only time that the conditions warrant the higher IDS pressure is when 100% of the nozzles fire. If the IDS pressure, however, is set to allow flow under the average use conditions, say 5% to 10%, then when the printhead fires a series of higher density images, the ink flow rate from the printhead would be insufficient. A printing system with a constant IDS pressure sets a pressure that is greater than the highest flow rate condition the printing system allows.
For these and other reasons there is a need for the present invention.